Field of the Invention
The present invention concerns a method for implementing a magnetic resonance examination of an examination subject in a magnetic resonance system with a predetermined magnetic resonance measurement protocol at several different bed positions of an examination bed of the magnetic resonance system. The present invention relates in particular to magnetic resonance whole-body examinations, such as for screening purposes, which are implemented with different magnetic resonance contrasts or magnetic resonance measurement protocols.
Description of the Prior Art
Magnetic resonance examinations can be used in the diagnosis of a multitude of illnesses. Furthermore, magnetic resonance examinations can be used as whole-body examinations for screening purposes, in order to search for sick persons in a defined section of the population within the field of preventative medicine, or to implement various subsequent examinations depending on the illness. A particular advantage of the magnetic resonance examination, in conjunction with screening purposes, is that the examination subject is not exposed to radioactive radiation during the examination. Nonetheless, the examination subject is exposed to electromagnetic fields during a magnetic resonance examination, and it is possible for these fields to be absorbed by biological tissue. The absorption of electromagnetic field energy may result in heating of the tissue and is therefore to be restricted during the implementation of a magnetic resonance examination. The specific absorption rate SAR is used as a measure of the absorption of electromagnetic field energy in biological tissue.
Whole-body examinations, as are used for screening purposes in conjunction with magnetic resonance examinations, may include magnetic resonance examinations with different magnetic resonance contrasts or according to different magnetic resonance measurement protocols. For instance, magnetic resonance measurement protocols can be used according to Turbo Inversion Recovery Magnitude (TIRM), Volume Interpolated Breath-Hold Examination (VIBE) or Diffusion Weighted Imaging (DWI), which are implemented at different bed positions and are then combined with one another.
FIGS. 1 and 2 show schematic representations of examination procedures of an examination of the thorax, abdomen and pelvis at different bed positions with different magnetic resonance measurement protocols. Each box in FIGS. 1 and 2 represents a corresponding magnetic resonance examination with the selected magnetic resonance measurement protocol at the selected bed position. The procedure of the whole-body measurement includes the measurement of different protocols and the repetition of these measurements at different bed positions. In this way, two different strategies can be used, which are shown schematically in FIGS. 1 and 2.
In the strategy shown in FIG. 1, which is also referred to as contrast-based strategy, each individual magnetic resonance measurement protocol or each individual magnetic resonance contrast is measured in sequence at all bed positions. The bed moves, for example, with N contrasts and M bed positions N×M times. The magnetic resonance measurement protocols of a contrast are executed directly one after the other. In FIG. 1, the bed is therefore moved at time instants t1, t2, t3, t4, t5, t6, t7, t8 and t9 respectively to the corresponding bed position and the corresponding measurement is implemented there, i.e. after t1, t2 and t3 respectively a DWI measurement, after t4, t5 and t6 respectively a TIRM measurement and after t7, t8 and t9 respectively a VIBE measurement.
Alternatively to the contrast-based strategy in FIG. 1, a region-based strategy can be implemented, which is shown schematically in FIG. 2. In each position, all contrasts or magnetic resonance protocols are measured in sequence and the bed is then moved to the next position, upon which all contrasts are measured again. The bed therefore only moves M times, namely at time instants t1, t4 and t7.
Despite the increased number of bed movements, the contrast-based strategy in FIG. 1 is generally preferred, since different protocols or sequences can use different fields of view, particularly in the z-direction, and the acquisition time can as a result be minimized such that the different protocols use different bed positions. This is symbolized by the different size of boxes in FIGS. 1 and 2. Furthermore, in the case of the contrast-based strategy, there is a lower probability that an examination subject moves between magnetic resonance examinations at the different bed positions, as a result of which errors in a combined overall tomography can be prevented. The region-based strategy in FIG. 2 requires the same bed positions for all contrasts, which is a necessary requirement particularly when magnetic resonance and positron emission tomography applications are combined.
Regardless of whether the contrast-based strategy or the region-based strategy is used, the aim is to obtain a combined image for the several bed positions for each contrast, for instance a combined image of the thorax, abdomen and pelvis. A requirement for this is that the magnetic resonance images recorded at the different bed positions can be combined suitably, that all measurements of a contrast use the same contrast-determining measurement parameters, such as for instance the same repetition times (TR), the same echo times (TE), the same tilt angles, etc. In order to ensure that the permissible specific absorption rates (SAR) are not exceeded, an SAR monitor is usually provided, which, prior to the start of a magnetic resonance examination at a specific bed position with a determined magnetic resonance measurement protocol, determines the SAR load of the examination subject to be expected. In the examples in FIGS. 1 and 2, the SAR monitor, at times instants t1, t2, t3, t4, t5, t6, t7, t8 and t9, determines the SAR load of the examination subject to be expected for the subsequent magnetic resonance examination respectively. Since the different body regions may have different SAR properties, this may result in the SAR monitor requiring an adjustment of the protocol parameters at individual bed positions, for instance a change in the repetition time TR, the tilt angle or the number of slices. This can result in the combined image indicating different contrasts, which may be shown as stripes in the image or can no longer be combined on account of reduced slices, since the body of the examination subject is no longer measured in detail.